The present invention relates to digital communications transmission services for multiple subscribers. Specifically, subscriber frequency control terminal equipment for transmitting and receiving data to and from a central station at microwave frequencies are described.
A large scale continental data communications system between central locations via satellite has been proposed by the telecommunications industry in a proposal before the Federal Communications Commission identified as a Pilot Demonstration Network. The system employs communications by satellite between central locations. Each central location has a capability of interconnecting a plurality of subscribers to the satellite through cable or by the use of TDMA and TDM radio services between subscribers and the central locations.
When using TDMA and TDM services, each subscriber is provided with a full duplex transceiver which operates in the 10 GHZ microwave frequency spectrum. The TDM signal sent to all subscribers in a given geographical area comprises packets of data information, which may be addressed to individual subscribers. The subscriber's transceiver identifies the packet contained in the TDM signal which is addressed to a specific subscriber. The identified data is decoded and inserted in memory for later use by the subscriber.
The subscriber may begin transmitting data in accordance with timing information contained in the received TDM signal. At the appropriate time, the subscriber transmits data to the central station on a microwave frequency which is offset from the TDM carrier signal frequency in a full duplex mode. When the TDMA signal has been transmitted its carrier signal ceases, providing an opportunity for transmission of yet another subscriber's TDMA signal at the same frequency.
In systems of this type, it is necessary to establish a TDMA signal carrier frequency which is maintained within a specified frequency tolerance between each subscriber station since the receiver frequency at the central location is fixed. Further, since the TDM and TDMA signals are simultaneous, it is necessary to maintain a predetermined frequency differential between the signals.
Various techniques for maintaining the required frequency tolerance for the TDMA signal are possible. However, most techniques require a high stability, local reference oscillator which controls the TDMA carrier frequency. The present invention seeks to provide TDMA carrier frequency control referenced to the TDM carrier signal frequency without the use of a highly stable local reference oscillator.